A HIGH TEMPERATURE FEED-THROUGH SYSTEM AND METHOD FOR MAKING SAME

Abstract: An electrical feed-through (100) for a conductor comprises: an electrically conductive base (112) adapted for attachment to a structure by a process utilizing heat; an electronic component (115) mounted in spaced relationship to the conductive base (112), the electrically conductive base and electrical component being separated by a gap; electrically conductive bridge means (130) comprising a thermal resistance greater than the thermal resistance of the electrically conductive base (112), the bridge means (130) bridging the gap between an electrically connecting the conductive base and the electronic component (115).

Description: A HIGH TEMPERATURE FEED-THROUGH SYSTEM AND METHOD FOR MAKING SAME. The present invention relates to electronic components, and, more particularly, an improved feed-through system for applications requiring the mounting of electronic components on or near the feed-through. Feed-throughs are well known in industry and are commonly used to provide electrically conductive paths through walls of articles. In many instances, unwanted high-frequency signals generated to one side of a wall pass through the feed-through and cause problems for circuitry situated on the other side of the wall. In an effort to alleviate interference from such sources, various filtering devices and/or techniques have been employed, such as ceramic capacitors. Feed-throughs with capacitors are intended primarily to pass relatively low frequency electrical signals via a lead or conductor through an opening in a conducting wall, screen or the like while presenting a low impedance shunt path to the conducting wall for high frequency signals. Capacitors of this type are disclosed in U.S. Patent Nos. 3,243,671 and 2,756,375. This type of feed-through arrangement is commonly used in electronic systems such as heart pacemakers, missiles, satellites and space probes. Illustrative are the feed- through capacitor systems disclosed in U.S. Patent Nos. 5,032,692, 4,424,551, 4,314,213 and 4,148,003. To maximize the effectiveness of such filter systems, the feed-through capacitor must be mounted as close to the feed- through as possible. The outside surface of the capacitor is typically covered by a metallic electrode that is connected to the container wall. By positioning the capacitor adjacent to the feed-through, the capacitor becomes a continuation of the container wall. In effect, the container wall ^covers' most of the insulator when the capacitor is so positioned. If the high-frequency filter is mounted on a circuit board in the container, the lead from the filter to the feed-through can act as an antenna which collects and transmits the high frequency electromagnetic radiation present in the container. Hence, the length of unshielded conductor from the final filter to the wall of the container must be minimized. One method for accomplishing this goal makes use of a feed-through capacitor mounted on the feed-through. The above described system requires that the capacitor be attached to the feed-through. One method for providing this attachment relies on bonding the capacitor to the feed-through using epoxy resin. This approach has a number of drawbacks. Firstly, epoxies are difficult to apply, particularly on small packages. This results in high labour costs and low yields. Secondly, epoxies are unstable at temperatures above 175°C. Such temperatures are often encountered when a feed-through assembly is attached to a container via welding or brazing. Thirdly, it is difficult to inspect the assembled part. One alternative to epoxy attachment is disclosed in U.S. Patent No. 5,032,692. In this system the capacitor attachment is achieved through the use of brazing alloy rings. The capacitor is brazed into a shielded compartment in the feed-through. While this method eliminates the use of epoxies, it requires capacitors that are capable of withstanding very high temperatures. The use of such capacitors increases the cost of the feed-through system. The present invention provides an electronic feed-through system for a conductor comprising:- an electrically conductive base adapted for attachment to a structure by a process utilizing heat; an electronic component mounted by electrically conductive bridge means in spaced relationship to the conductive base, the electrically conductive base and electrical component being separated by a gap; the electrically conductive bridge means having a thermal resistance greater than the thermal resistance of the electrically conductive base, and bridging the gap between and electrically connecting the conductive base and the electronic component. Further features and advantages will become apparent from the following description of embodiments of the invention, as illustrated in the accompanying schematic drawings, in which: Fig.l is a cross-sectional view of a prior art feed- through system. Fig. 2 is a cross-sectional view of a feed-through system according to the present invention. Fig. 3 is a bottom view of a feed-through system according to the present invention. Fig. 4 is a bottom view of another embodiment of a feed-through system according to the present invention. Fig. 5 is a cross-sectional view of another embodiment of a feed-through system according to the present invention. Fig. 6 is a cross-sectional view of another embodiment of a feed-through system according to the present invention. Fig. 7 is a cross-sectional view of another embodiment of a feed-through system according to the present invention. The present invention will be described with reference to a hermetically sealed electronic feed-through system utilizing a feed-through capacitor. It will, however, be recognized by one skilled in the art that the invention may be employed in feed-through systems which do not require hermetic sealing and/or which utilize other electronic or heat sensitive components. The manner in which the present invention achieves its advantages over the prior art may be more easily understood with reference to Fig. 1 which is a cross-sectional view of a typical prior art feed-through system 10 utilizing epoxy bonding. The feed-through system 10 generally comprises a housing 12, an electrical conductor 13 extending through a hole in an insulator 14. A feed-through capacitor 15 is disposed within the housing 12. An epoxy layer 16 is employed to secure the feed-through capacitor into housing 12. Feed-through capacitor 15 has two electrodes, electrode 17 on an inside surface providing electrical connection to conductor 13 and electrode 18 on the outside surface providing electrical connection to housing 12. After assembling feed-through system 10, it is typically attached to the wall 20 of a container by high temperature brazing or welding at perimeter 21. The temperatures at perimeter 21 can result in a significant transfer of heat along the walls of housing 12 as shown by the arrows in Fig. 1. The amount of heat actually transferred from perimeter 21 to the epoxied feed-through capacitor 15 can be sufficient to damage feed-through capacitor 15 or to cause the epoxy bond to fail. This problem is particularly acute in systems in which the feed-through system has a small thermal mass. The present invention utilizes an electrically conducting bridge of high thermal resistance to connect the feed-through capacitor to the wall of the feed-through housing while limiting the amount of heat that can flow from the feed- through wall to the capacitor during the attachment of the feed-through system to the container. The present invention also avoids the necessity of using epoxy resins. Refer now to Fig. 2 which is a cross-sectional view of a feed-through system 100 according to the present invention. [In Fig. 2 and subsequent Figs, elements that serve analogous functions to elements shown in Fig. 1 have been assigned numbers that differ from those in Fig. 1 by 100,200, etc.] Feed-through system 100 provides a conducting path for conductor 113 through the wall 120 of a container via insulator 114. Feed-through system 100 is adapted for connection to wall 120 by welding of housing 112 to the wall 120. Housing 112 and electrically conducting bridge 130 provides the electrical connection between wall 120 and the outer surface electrode of feed-through capacitor 115. Capacitor 115 has a second electrode disposed on the inside surface of region 117 that is connected to lead 113 by the application of a conductive bonding agent. Bridge 130 has three regions. The first region 131 connects the bridge to housing 112. The second region 133 connects the bridge to feed-through capacitor 115. Between regions 131 and 133 the third region 132 provides a thermally resistant electrical connection between regions 131 and 133. In the preferred embodiment of the present invention, the cross-sectional area of region 132 is considerably less than the cross-sectional area of housing 112. Hence, the thermal resistance of region 132 is substantially greater than that of housing 112. The region of increased thermal resistance impedes the flow of heat from housing 112 to feed-through capacitor 115 during the attachment of feed-through system 100 to container 120. Bridge 130 may be constructed from any electrically conducting material. In one embodiment of the present invention, the housing 112 is made of titanium and the bridge 130 is constructed from a nickel ribbon. In the preferred embodiment actual dimensions of the housing and bridge components are not as important as their thicknesses. Preferably the bridge thickness is 20% or less of the housing thickness. The selection of an appropriate bridge thickness and material includes considerations of, among other criteria, cost, manufacturability, and solderability. In general terms, Fourier's Law defines the heat flow. k A ΔT Fourier's Equation q = q = heat flow k = thermal conductivity A = cross sectional area of bridge 1 = length of bridge ^ = temperature differential between housing and bridge. It is clear from the equation that minimal heat flow will be accomplished by minimizing the conductivity of the bridge and its cross sectional area. The length of the bridge and the heat exposure are set by the required dimensions and uses of the feed-through. It is clear that if 1 is zero (effectively the situation in the prior art feed-throughs) there will be such high heat flow that ΔT will be effectively zero. Accordingly 1 should be as large as is feasible given the dimensional constraints of the feed-through and component. The cross-sectional area A of the bridge will be determined both by overall geometry of the bridge and by the thickness of the bridge material. In practice variation of these parameters will give wide variation in the heat flow through the bridge. This is made clear in the following descriptions of preferred embodiments. In a preferred embodiment of the present invention, the bridge completely surrounds the feed-through capacitor as shown in Fig. 3 which is a bottom view of a feed-through system 200 according to the present invention. A bridge comprising regions 231-233 is electrically connected and bonded to housing 212 of feed-through system 200 by bonding region 231. The bridge is electrically connected and bonded to the outer electrode of feed-through capacitor 215 by bonding region 233. Region 232 of the bridge provides a thermally resistant electrical connection between regions 231 and 233. Feed-through capacitor 215 has a hole therein for connection to lead 213. The arrangement shown in Fig. 3 is preferred because the bridge provides a shield against electromagnetic radiation. That is, there is no possibility of electromagnetic radiation passing between feed-through capacitor 215 and housing 212. However, this arrangement does not minimize the cross-sectional area of region 232; hence, it does not provide the maximum thermal isolation. A bottom view of a second embodiment of the present invention that provides improved thermal isolation is shown in Fig. 4 at 300. In this embodiment, the bridge region 332 consists of thin ribbon extending from the housing 312 of the feed-through to the outer electrode of feed-through capacitor 315. The centre electrode of feed-through capacitor 315 is connected to conductor 313. The thermal resistance of region 332 may be controlled by changing the cross-sectional area of the region and/or by changing the material from which the region is constructed. The cross-sectional area may be reduced by cutting notches in region 332 to form a small restricted bridge region as shown at 355. Similarly, region 332 or region 355 could be constructed from a material having a higher thermal resistivity than housing 312. The connection to feed-through capacitor 315 may be made with the aid of spot welds such as those shown at 350. For the purpose of this discussion, the term spot weld' shall be defined to include any method of attachment in which a small conducting area is created between two surfaces. Small areas of brazing alloys or other conductive bonding agents may be present between the two surfaces. The spot welds further limit the cross-sectional area of the bridge that is available to transfer heat to feed-through capacitor 315. By bonding with an alloy that has a sufficiently low melting temperature, the bond may be used as a *thermal fuse' that will break if the feed-through is overheated during attachment to the container. Visual inspection of the bond can then be used to identify components that were overheated during assembly. Thus, parts that might fail in service may be eliminated before being placed in service. It should also be noted that the spot welds may be introduced between feed-through housing 312 and the portion of region 332 that is adjacent thereto. This method of spot welding to provide a thermal fuse provides some additional advantages, since the bridge has a high thermal resistance the temperature at which the weld is designed to fail may then be greater than that of the weld shown at 350. There are more choices of alloys for the spot welds if the failure temperature is higher. It should also be noted that the thermal fuse can be provided by region 332 or region 355. This may be accomplished by utilizing a material that melts when subjected to heating beyond a predetermined temperature. The feed-through capacitor may be positioned adjacent to the insulator 114 as shown in Fig. 2 or it may be spaced therefrom as shown in Figs. 5 and 6. Referring to Fig. 5, a spacer 540 may be used to set the distance between the feed- through capacitor shown at 515 and the insulator. Referring to Fig. 6, if the space 645 between feed-through capacitor 615 and insulator 614 is not filled by a spacer, one or more ports such as port -646 accessing the portion of lead 613 within space 645 may be provided. This port may be used to visually inspect lead 613 or make test connections thereto. Such ports may also be used to inspect the insulator 614 during testing of the hermetic seal or to inspect the hidden side of capacitor 615. The configuration shown in Figs. 4-6 also allows for the use of a flexible bridge to accommodate any thermal expansion mis-match during the attachment of the feed-through system to the container. While the above described embodiments of the present invention have shown a feed-through capacitor housed within a feed-through housing, other configurations will be apparent to those skilled in the art. Referring again to Fig. 2, feed- through housing 112 provides structural support for attaching bridge 130 and, in combination with bridge 130, provides a ground shield that isolates the portion of lead 113 that passes through insulator 114. In addition, the ground shield prevents electromagnetic radiation from escaping from the container via insulator 114. However, bridge 130 may also provide these functions as shown at 700 in Fig. 7. In this embodiment of the present invention, bridge 730 connects to feed-through base member 712 which does not house feed-through capacitor 715. Bridge 730 provides the electrical shielding of feed-through capacitor 715 and lead 713. The construction of bridge 730 provides a very long pathway for heat transfer (1 in Fourier's equation is large) and so reduces heat flow considerably. Although the above described embodiments of the present invention are directed to a feed-through system with one lead, it will be apparent to those skilled in the art that systems having a plurality of leads may also be constructed according to the present invention. It will also be apparent to those skilled in the art that the present invention may also be used to construct feed-through systems in which the feed-through is not cylindrical in shape. Similarly, it will be apparent to those skilled in the art that the feed-through capacitor described in the above embodiments of the present invention could be replaced with any electrical component that must be positioned close to the container wall and connected thereto.

Claims: CLAIMS: . 1. An electrical feed-through for a conductor comprising:- an electrically conductive base adapted for attachment to a structure by a process utilizing heat; an electronic component mounted by electrically conductive bridge means in spaced relationship to the conductive base, the electrically conductive base and electrical component being separated by a gap; the electrically conductive bridge means having a thermal resistance greater than the thermal resistance of the electrically conductive base, and bridging the gap between and electrically connecting the conductive base and the electronic component. 2. An electrical feed-through as claimed in claim 1 in which the bridge has a cross-sectional area 20% or less that of the base. 3. An electrical feed-through as claimed in claim 1 or claim 2 in which the base comprises a housing within which the electrical component is mounted. 4. An electrical feed-through as claimed in claim 3 in which the housing comprises an aperture having a mouth, the component lying within the aperture, the bridge, component and conductor completely closing the mouth. 5. An electrical feed-through as claimed in any of claims 1 to 3 in which the bridge comprises a ribbon. 6. An electrical feed-through as claimed in any preceding claim in which the bridge means is connected to the electrically conductive base and/or to the electrical component by spot welds. 7. An electrical feed-through as claimed in any preceding claim and comprising a thermal fuse. 8. An electrical feed-through as claimed in Claim 7 in which the bridge comprises the thermal fuse. 9. An electrical feed-through as claimed in claim 8 in which the bridge is a ribbon and the thermal fuse comprises a constriction in the ribbon. 10. An electrical feed-through as claimed in any preceding claim in which the electrically conductive base houses an insulator held in spaced relationship to the electronic component. 11. An electrical feed-through as claimed in Claim 10 in which an insulating spacer is disposed between said insulator and electronic component.

Inventors: BEALKA DAVID JOSEPH, BROWN TIMOTHY JAMES, SHIPMAN THOMAS WAYNE, BEALKA, DAVID, JOSEPH, BROWN, TIMOTHY, JAMES, SHIPMAN, THOMAS, WAYNE
Applicants: BEALKA DAVID JOSEPH, BROWN TIMOTHY JAMES, MORGAN CRUCIBLE CO, SHIPMAN THOMAS WAYNE, BEALKA, DAVID, JOSEPH, BROWN, TIMOTHY, JAMES, SHIPMAN, THOMAS, WAYNE, THE MORGAN CRUCIBLE COMPANY PLC
Application year: 1994
Main classification: H01G4
IPC codes: H01G2, H01G4